Glass electrode and method



Jana, 1945. l H, H, @RY -TAL 2,366,489

GLAss ELEcTRoDE AND METHOD Filed Aug.` 2.1 1940 #aff/5, Hic/7; faam-:ee ww@ Patented Jan. 2, 1945 UNITED STATES PATENT OFFICE Henry H. Cary,

Baxter, Pasadena, Calif., assignors to Technical Laboratories, South Pasadena,

South Pasadena, and Warren P.

National Calif.,

a corporation of California Application August 2, 1940, Serial No. 349,858

26 Claims.

This invention relates to the measurement of pH by means of glass electrodes, and relates more particularly to a conditioning treatment for the electrode surfaces of a glass electrode.

As ordinarily constructed, a glass electrode consists of a thin bulb of special low resistance electrode glass blown on the end of a supporting tube composedof a glass having a high electrical resistance. When solutions of different pH are placed in Contact with the inner and outer walls of the bulband proper electrical precautions are taken, it is found that an electric potential difference exists between the two solutions. This potential difference Avaries with changes in pH of the inner and outer solutions and may be expressed by the following formula over a limited range of pH values:

E=A+ (.00019832)T(pH1-pH12) Where E is the potential of the inner solution with respect to the outer, pH1 is the pH of the after a few days boiling the usual electrode will not correctly indicate vpH unless an undue time is allowed for it to come to equilibrium. In the value from a few millivolts to twenty millivolts or more and is commonly referred to as the asymmetry potential of the electrode.

The observed potentialE may be regarded as having its origin in the difference between a potential arising at the reference electrodeV surface which is in Contact with the reference solution and a potential arising at the test electrode surface' which is in contact with the test solution. Thus viewed. the asymmetry potential as represented by the term A is apparently related to diierences in activity asA between the reference and test electrode surfaces of thevglass electrode.

The asymmetry potential of the glass electrode introduces a complication in practical pH measurement since it is necessary to evaluate it in some manner before-the potential measurements of the electro-de can be interpreted in terms of the pI-I of the solutions with which it is in contact. In practice, this usually is accomplished by calibrating the electrode against solutions of accurately known pH. This calibration must be repeated frequently since` as indicated, the asymmetry/ potential of an electrode usually `changes with age.

past, when an electrode had reached a certain degree ofsluggishness its usefulness was at an end and it was necessary to discard. it. This has greatly hindered the application of the glass yelectrode in many industries where the pH measurements musty be made on hot solutions and the replacement cost of the electrodes would therefore be prohibitive.

It is an object of the present invention to provide a method for conditioning a glass electrode whereby one or vboth electrode surfaces may be brought to or maintained at a basic level of activity and/or metry potential, sluggishness, and the deleterious effects of aging in general may beminimized or overcome. A further object is to reduce the errorsv of pH measurement arising from the aging of the glass electrode.

Another object is to provide glass electrodes of such low asymmetry potential that different electrpdes may be interchanged without requiring recalibration.

A further object is to provide a method and apparatus for storing'intermittently used glass electrodes to overcome the effects of usage and aging and to maintain the electrode in a continuously useful condition. Y

It is a further object to -provide a glass electrode, particularly of the sealed type, in which the aging effects at the inner electrode surface are continuously and automatically minimized or suppressed.

Further objects and features will be apparent from the following detailed description.

The gure of the accompanying drawing is a vertical cross-sectional view of one embodiment of a glass electrode, including a body or supporting tube i0 on which is blown a thinbulb Il of electrode glass providing an inner or reference' surface I4 and an outer or test surface I5. 'Ihe response, and whereby the asym.

ample, by an electrode wire shaped and sealed in a manner similar to the showings of our Patent No. 2,256,733. The invention is applicable tovarious types and shapes of glass electrodes suitable for measurement of pH, the figure being merely exemplary.

The present invention, which offers a solution to the above-described difficulties and which permits the attainment of the stated objects, is*

based, in part, on `our discovery that the sluggishness and asymmetry vpotential-associated with Using a paraflln lined beaker, a solution of hydrofluoric acid is prepared by diluting-the commercial 45% acid with two parts of water. A portion of the acid solution is poured into the electrode body and the bulb is immersed in the remainder of the solution so that etching takes place on the inner and outer surfaces of the bulb. After etching for approximately one minute, the electrode is rinsed in water and allowed to soak for a few hours in a dilute acid solution such as 0.1 N HC1 to stabilize its characteristics. On testing, the electrode will then be found to have essentially zero asymmetry potential and to re spond to pH changes with the same rapidity as a freshly made electrode. The figure of the accompanying drawing suggests, by legends, such chemically-etched reference and test surfaces resulting from `internal and external contact of the electrode bulb Il 'with a. chemical etching agent. capable of dissolving the glass of the bulb.

In some respects it is preferable to employ ammonium bifluoride instead of hydrofluoric acid as the etching agent. 'Ihis salt is ymoreconvenient and less dangerous to handle than the hydrofiuoric acid. Its action is somewhat slower, however, a solution requiring about two minutes to etch the usual aged electrode.

It is to be noted that the amount of glass removed from the electrode by these procedures is quite small-it is insufficient to appreciably change the electrical resistance of a rugged glass electrode bulb-and therefore this renewal process may be repeated vmany times on the same electrode. However, the electrode bulb should be well annealed if many renewals are-to be made, as otherwise the release of surface strains by the etching may cause the bulb to break.

In addition to the feature of renovating aged electrodes, our invention further extends to the maintenance of an electrode in goo'd condition. When a glass electrode is only in intermittent use, this feature of maintenance may be realized by storing the electrode in 'anetching solution that is very slow in its action. By proper choice of solution the etching may be made .to take place at about the same rate as the aging and thereby maintain the electrode in a continuously serviceable condition. For this purpose we und advantageous a dilute solution of a' 'non-etching acidv Isaturated with a slightly soluble fluoride salt, for

example, a solution containing 0.1 N hydrochloric acid and an excess of solid calcium fluoride. 'I'he calcium fluoride. being only slightly For badly aged electrodes, we use thel l following procedure:

, l 2,806,459 any suitable means known in the art,for ex soluble, continuously maintains' a minute concentration of hydroiluoric acid in the solution. At

this low hydrofluoric acid concentration etching takes piace very slowly, only slightly faster than the rate of aging of the glass. By adding a soluble calcium salt, like calcium acetate, tothe solution or by reducing the hydrochloric acid concentration, the action may be made even slower.,

decreases with decreasing pH. At a pH of about 9.5 the etching action occurs at a rate only slightly greater than the rate of aging. When stored in solutions'of this pH, 'glass electrodes show no appreciable change in asymmetry potential even at elevated temperatures.

Alkaline solutions are particularly useful for storing and renewing electrodes that are to be used mainly for measuring the pH of alkaline solutions. Besides preventing the formation of asymmetry potentials, ,i the alkaline storage solution conditions the electrode so that it rapidly attains equilibrium when pH measurements are made in other alkaline solutions. We prefer a borate buffer of pH 10 as an alkaline solution in which to store electrodes; however, we flnd that satisfactory freedom from asymmetry potential formation may be had in any solution above DH 8.0.

Solutions havingva pH'value above 11.0 havea relatively rapid etching action and such solutions may be used for reconditioning aged electrodes. Thus, a 1 normal sodium hydroxide solution will reduce the asymmetry potential of a badly aged electrode to a low value in about four days at room temperature.

A further important feature of our invention arises from our discovery that the same etching agents as described above will also reduce the reli ativelysmall asymmetry potential of freshly made glass electrodes to zero. Although the asymmetry potential of a freshly made electrode may only be a few millivolts, it is large enough to be troublesome in manufacture, since it necessitates calibrating individual electrodes before use.' Electrodes treated with an etching agent are made interchangeable and the preliminary posed to the same pH, sluggishness of response is of no consequence at the inner surface, and

satisfactory performance as regards rapidity of response may be obtained from aged electrodes by etching the outside surface only. In general, this procedure does not reduce the asymmetry potentialto zero: in fact, it usually increases it as the usual tendency is'forthe contributions of the two surfaces to the asymmetry potential to balance each other. In consequence, etching one surface only will not eliminate the need for a caliascenso bration', but it will improve the speed of response of the electrode to solutions in contact with its etched surface.

In many commercial glass electrodes, the internal solution is permanently -sealed in plaee,as,-

struction may be renewed when sluggish by etching the outside surface only. However, as explained above, this procedure usually does not reduce the asymmetry potential to zero as the contribution to the asymmetry potential associated with the inner surface of the electrode remains4 unaffected. In order to keep the inner surface of the electrode at a basic level of activity corresponding to that of a freshly etched surface we employ a filling solution compounded to have a slow etching action and with this modification the asymmetry potential of the electrode as a whole may be reduced to zero at any desired time simply by etching the outer surface. It willbe apparent that with this combination the electrode may be completely renovated as regards both sluggishness and asymmetry potential, or may be maintained continuously serviceable during storage, without the necessityof breaking the seal.

An acid filling solution having a slow etching action may be made by using slightly soluble iiuorides as described in connection with the storage of glass electrodes. For example, the filling solution may consist of 0.1 N hydrochloric acid saturated with calcium fluoride, the electrical connection to the solution being made by means of a sealed-in silver-silver chloride electrode. However, such solutions are not entirely satisfactory for this purpose since the acid concentration changes slowly as the etching proceeds andthe filling solution cannot be depended upon to maintain a constant pH'over a long period of time as is necessary if recalibration is to be avoided.

In permanently sealed glass electrodes, we prefer to use an alkaline solution to obtain the etching action. As mentioned previously. electrode aging takes place very slowly.' if at all,-when the pH is above 8.0. Furthermore, we find that the pH ofthe etching solution in contact with the glass will not change appreciably if its pH is in the vicinity of 9.5 so that a 'solution in this pH range has the desirable characteristics of a slow etching action and a stable pH. Although the pH of 9.5 appears to beoptimum, the particular value used for the filling solution does not seem to be critical and satisfactory results in suppressing aging are obtainable lwith solutions having a range of pH values extending from 11.0 to 8.0` or lower.

In accordance with' these principles we have constructed glass electrodesy similar to those described in our lpatent mentioned above in which the filling solution was a borate buffer of pH 9.2

containing potassium chloride in a concentration of 0.25 mol per litre, electrical connection to the solution being made by means of a sealedin silver-silver chloride electrode. `Tests demonstrated that inappreciable aging occurred at the inner surface of the glass electrode and that the pH of the filling solution remained essentially constant even afterlong exposure to elevated temperatures.

It -will be understood that the details ofthe above description are intended as exemplary rather than limiting, and various modifications will be apparent to one skilled in the art as em'` bodying the assence of our invention as defined by the scope of the appended claims.

We claim as our invention:

1'. A method of manufacture of glassl mem.

said etching agent from contact with said membrane before the etching action has appreciably changed the electrical resistance of said membrane and when the asymmetry potential of the membrane has beenereduced substantially to zero.

2. A method of renovating an aged glass electrode, which electrode has a relatively high asymmetry potential which has increased because of use, to reduce said asymmetry potential substantially to zero, and which glass electrode has two electrode surfaces, which method includes the steps of contacting said two electrode surfaces with a chemical etching agent capable of dissolving the glass of said electrode; and maintaining said contact between -said chemical etching agent and each of said surfaces only for a period of time sufficient to dissolve a superficial layer of said glass from each of saidsurfaces and insuilcient to dissolve such large quantities of glass as would produce a substantial decrease in the electrical resistance of saidv e1ectrode,.said period of contact being sufficient to reduce the asymmetry potential of said glass electrode substantially to zero.

maintaining said electrode in good condition, which method includes the steps of: `storing `between uses a glass electrode, which has the inner surface thereof chemically etched, in a solution of a chemical etching agent capable of dissolving superficial 4layersof glass from the outer surface of said electrode,.the strength of said etching solution being adjusted with reference to the rate of aging to remove glass from said surface electrode having a reference surface and a test surface, which ,method includes the stepsl of: maintaining said' reference surface continuously in contact witha reference solution consisting of 'a chemical Vetching solution of such composition and strength' as to slowly etch said reference surface by dissolving glass therefrom and of such composition and strength that such etching takes place at substantially the same rate as said aging to suppress said aging and maintain said reference surface at a basic level of activity which is substantially constant from time to time; and periodically contacting said test surface with a chemical etching agent capable of dissolving glass, while maintaining said contact between said chemical etching agent and said test surface 3. Amethod for preventing aging of a glassI `electrode subject to intermittent use and for only for a period'of time suiiicient to dissolve a superficial layer of said glass from said test surface and to reduce the asymmetry potential of said electrode substantially to zero.

7. A method as dened in claim 6, in which said reference solution`is an alkaline solution having a `DH between about 8 and 11.

8. A method as defined in claim 6, in which said reference solution isbuiered at a pH of approximately 9.5. y

9. A method as defined in claim 6, in which said reference solution is a dilute non-etching acid saturated with a slightly soluble fluoride salt.

10. A glass electrode having low-aging characteristics, which electrode includes 'in combination: a glass electrode bulb providing an inner reference surface and an cuter test surface; and a reference solution in contact Withsaidrrefer-l ence surface and comprising a chemical etching .Y

solution capable of slowly dissolving glass from said reference surface, said chemical etching solution being of such composition and strength that said etching takes place at about the same rate as the aging of said electrode.

1l. A glass electrode of substantially zero asymmetry potential and which will respond rapidly when immersed in solutions to be tested, said glass electrode including in combination: a glass electrode bulb' providing an inner reference surface and an outer test surface: and a reference solution in contact with said reference surface and comprising a chemical etching solution capable of slowly dissolving glass from said reference surface, said chemical etching solution being of such composition and strength that said etching takes place at about the same rate as the aging of said electrode, said outer test surface being chemically 1etched in sufficient amount to bring the asymmetry potential of said electrode substantially to zero.

ically-etched on both said reference and test surfaces in suiiicient degree to reduce the asymmetry potential substantially to zero.

1'?. The method of producing a glass electrode which hasl substantially zero asymmetry poten...

tial, which method comprises: subjecting a glass electrode, which has one surface thereof chemically etched, to an etching operation comprising contacting the other surface of said electrode with a chemical etching agent to remove a superiicial layer of the glassof said other surface so 12. A. glass electrode as dened in claim 11, c

en ce surface is slowly etched while the pH of the l reference solution remains substantially constant.

Y 15. A combination as dened in claim 11, in which the reference solution is alkaline and has a pH within the range of substantially 8 to 11,l

said, reference solution containing a buffer acting to maintain the pH of the reference solution substantially constant within said pH range of substantially 8 to 11.

16. -A glass electrode providing an inner refer.. ence surface and an outer test surface and having substantially zero asymmetry potential, which glass electrode includes a glass membrane chemas to reduce the asymmetry potential of said glass electrode to substantially zero.

18.` The method of reducing the asymmetry potential of a glass electrode to substantially zero, which comprises: subjecting a glass electrode, 'which has the inner surface thereof chemically etched,l to an etching operation'comprising contacting the outer surface of said electrode with a chemically etching agent lto remove a superficial layer of the glass of said outer surface so as to reduce the asymmetry potential of said glass electrode to substantially zero.

19. The method of reducing the asymmetry potential of a glass electrode to substantially zero, which comprises: contacting one surface of said electrode with a chemical etching agent to remove al layer of glass from said surface; and contacting the 'other surface of said glass electrode with a chemical etchingagent to remove a superficial layer of the glass of said other surface so as to reduce the asymmetry potential of said glass electrode to substantially zero.

20. The method of renovating an aged glass electrode, which electrode issluggish in action because ci use, which method includes the stepy of: subjecting a glass electrode, which has one surface thereof chemically etched, to an etching operation comprising contacting the other surface of said electrode with a chemical etching agent to remove a. superficial layer of the glass of said other surface so as to reactivate said electrode and reduce thev asymmetry potential of said glass electrode to 'substantially zero.

2l. A method as defined in claim 18 in which the chemical etching agent is an alkaline solution having a pH between about 8 and about 11.

22. A method as defined in claim 18 in which the chemical etchingagent is an ammonium .biuoride solution.

23. A method as defined in claim 18 in which the chemical etching agent is an acid fluoride solution.

24. A method as defined in claim 20 in which the chemical etching agent is an alkaline solution 4having a pH between about 8 and about 11.

25. A method as defined in claim 20 in which the chemical etching agent is an ammonium bifluoride solution. A

26. A method as defined in claim 20 in which the chemical etching agent is ari acidiiuoride solution.

HENRY H. CARY. WARREN P. BA. 

